With the continuous development of society, automobiles become increasingly popular. Driving makes it convenient for people to travel, but also has negative impact on human society. A rapid increase in a quantity of vehicles causes a series of problems such as urban traffic congestion, frequent traffic accidents, and environmental degradation. According to statistics, nearly 200,000 traffic accidents occur in China in 2013 with a death toll of 58,000 and a direct economic loss of 1.04 billion yuan. A perfect intelligent transportation system (ITS) is desired from many angles such as personal safety, traffic efficiency, environmental protection, and economic benefits. Currently, the ITS has naturally become a focus of global attention.
At present, a vehicle may obtain information about road conditions or receive an information service in time by means of vehicle-to-vehicle (V2V) communication or vehicle-to-infrastructure (V2I) communication. Specifically, the vehicle may broadcast information such as a vehicle speed, a heading direction, and a specific location of the vehicle and information about whether brakes are slammed on to a surrounding vehicle by means of V2V communication. A driver can better know traffic conditions beyond a line of sight after obtaining the information, so as to predict and avoid a dangerous situation. During V2I communication, in addition to exchange of the foregoing safety information, roadside infrastructure may further provide the vehicle with all kinds of service information and an access to a data network. Functions such as electronic toll collection and in-vehicle entertainment greatly improve traffic intelligence. A network used by V2V/V2I communication is referred to as the Internet of Vehicles.
Long Term Evolution (LTE) is a mainstream wireless communications technology at present. A device-to-device (D2D) technology is considered as an important characteristic and is standardized in the 3rd Generation Partnership Project (third Generation Partnership Project, 3GPP) LTE R12, and supports direct communication between user terminals. Considering that a V2V/V2I communication scenario also belongs to terminal direct communication, the D2D technology may be used for V2V/V2I service transmission.
In an existing LTE R12 D2D communications technology, if a terminal can find a cell on a D2D transmission carrier frequency of interest, it is considered that the terminal is within a network coverage area. Otherwise, it is considered that the terminal is beyond a network coverage area. The terminal within the network coverage area implements synchronization by using a synchronization signal of a base station. The terminal beyond the network coverage area needs to search for a surrounding synchronization source before performing D2D communication. If a synchronization source satisfying a signal quality requirement can be detected around, the terminal synchronizes with the synchronization source. Otherwise, the terminal serves as a synchronization source to determine a synchronization signal sending moment and send a synchronization signal at the corresponding moment. If a receiving terminal finds multiple synchronization sources, on the premise that the multiple synchronization sources satisfy the signal quality requirement, priorities for accessing different synchronization sources are distinguished as follows: A synchronization source within the network coverage area has priority over a synchronization source beyond the network coverage area.
In the prior art, when a terminal beyond a coverage area sends a synchronization signal as a synchronization source, different synchronization sources are probably asynchronized, and asynchronization between two synchronization sources causes asynchronization between receiving terminals covered by the two synchronization sources. Consequently, communication between the two receiving terminals is affected, and a problem of inability to communicate with each other or a miss in reception may occur, severely affecting communication performance.